In the past few decades, telephone systems have evolved continuously at an accelerated rate. Networks operating under the Universal Mobile Telecommunications System (“UMTS”) Long Term Evolution (“LTE”) standard, initiated by the third-generation partnership project (“3GPP”), include new radio access technology (“RAT”) and core radio network architecture that provide high data rate, low latency, packet optimization, and improved system capacity and coverage. In LTE networks, an evolved universal terrestrial radio access network (“EUTRAN”) includes a plurality of evolved Node-Bs (“eNode B”s) and communicates with a plurality of mobile stations, also referred to as user equipments (“UE”s).
Intersystem handovers between LTE networks and Universal Terrestrial Radio Access Networks (“UTRAN”) are expected to play an increasingly important role as many cellular network operators will likely provide wide-area coverage with UTRAN and use LTE in relatively small traffic hotspots. This will be particularly common as LTE networks are implemented and expanded. A multisystem UE, e.g., a mobile phone or other wireless device, is capable of working on both LTE and UTRAN systems. Each time the UE moves between the two types of coverage areas, an intersystem handover is performed. These handovers are generally referred to as inter-Radio Access Technology (“inter-RAT”) handovers. An inter-RAT handover to UTRAN procedure hands the UE over from an LTE system to a UTRAN system.
Single Radio Voice Call Continuity (“SRVCC”) provides voice call continuity in a handover between a packet-switched (“PS”) network and a circuit-switched (“CS”) network for calls that are anchored in the IP Multimedia Subsystem when the UE is capable of transmitting/receiving on only one of those access networks at a given time. To perform an SRVCC handover from LTE to UTRAN, the LTE network sends a Handover to UTRAN Command Message to the UE.